We have recently started using IIS 7.0 in our test environment when we deployed our application that uses the BotDetect Captcha control the image stopped being displayed.
So far we have found one way of fixing this, we switched the default Integrated Application Pool Pipeline Mode over to Classic.
To do this simply go into the properties of the Application Pool in which your web application resides inside and change the setting, refresh your page and bingo you should see the Captcha image.
I was just looking at the technical FAQ on the Lanap software website and they suggest that you can make changes in the web.config to achieve the same result without moving to Classic mode. Below is there suggestion I have not tested this but expect is should work.
<validation validateIntegratedModeConfiguration="false" />
<add name="LanapCaptchaHandler" preCondition="integratedMode"
type="Lanap.BotDetect.CaptchaHandler, Lanap.BotDetect" />